(1) Field of the Invention
The invention relates to the general field of chemical mechanical polishing (CMP) with particular reference to the problem of dishing during shallow trench isolation.
(2) Description of the Prior Art
As part of the process of manufacturing integrated circuits, it becomes necessary to electrically isolate various active devices one from another. Probably the most widely used method is Local Oxidation of Silicon (LOCOS) wherein selected regions on the semiconductor surface are oxidized to substantial depths to form regions called field oxide. As circuit densities grow ever greater, this method is limited primarily by the fact that the boundary between oxide and semiconductor tends to have a relatively gentle slope with respect to the semiconductor surface. Additionally, since the field oxide extends above the surface as well as below it, it will eventually need planarizing.
To overcome these limitations, the method of shallow trench isolation (STI) has been developed. In STI, a trench, having near vertical sides, is etched out of the semiconductor and then filled with insulating material. The excess filler material is then removed by CMP. A serious problem associated with this isolation technique is that during CMP dishing of the surface of the filled trench occurs.
To illustrate this, we refer now to FIG. 1a which is a schematic cross-section of part of a silicon substrate 1 on which thermal oxide layer 2 is covered by silicon nitride layer 3. The latter has already been patterned and etched, using standard photolithographic techniques, to serve as a mask during the etching (usually reactive ion etching) of trench 5. The latter is then over-filled with a layer of insulating material such as silicon oxide 4 deposited through chemical vapor deposition (CVD oxide).
The excess of filler material 4 is then removed by CMP. It is often the case that the removal rate (by CMP) of the filler material 4 is significantly greater than the removal rate of trench masking material 3. To meet subsequent processing requirements, it is necessary that all of layer 4 (other than that which is filling trench 5) be removed. As a result, as illustrated in FIG. 1b, the surface 6 of trench 5 develops a concave aspect called dishing because too much of layer 4 will have been removed.
Most systems provide means for detecting when, in the course of CMP, layer 3 has been reached. This is important to ensure that, while layer 4 has been fully removed, as little as possible of layer 3 gets removed. To this end a substantially different removal rate for layer 4 relative to layer 3 is desirable. In practice, this necessary difference can only be achieved by making the removal rate for layer 4 faster than that of layer 3. There are thus contradictory requirements for relative removal rates between layers 3 and 4 which make it difficult to remove only filler material, and no more, while at the same time avoiding dishing effects.
There have been a number of proposals offered in the prior art to deal with this problem. For example, Morita (U.S. Pat. No. 5,540,811 July 1996) describes a method that relies on the selective placement of areas comprising slow polishing material over convex (projecting) areas of fast polishing material for their protection. Burke et al. (U.S. Pat. No. 5,356,513 October 1994) lay down alternating hard and soft layers, the minimum configuration being hard-soft-hard or soft-hard-soft. During polishing, the initial removal of hard material is from the high spots, the soft material being thereby exposed being then rapidly etched while hard material still present over the low spots etches slowly. This leads to a general flattening of the surface. This invention is intended for use in planarizing the surfaces of via holes filled with tungsten. Since tungsten is hard, dishing is not a problem.
Dawson et al. (U.S. Pat. No. 5,516,729 May 1996) teach a method of planarizing based on modifying the hardness of their top surface through heat treatment, prior to the application of CMP. Pasch (U.S. Pat. No. 5,441,094 August 1995) is similar to Burke et al. (supra) in that a hard cap layer is also placed over the trench filler material. As polishing proceeds, the cap layer is selectively removed from the high spots so the now exposed filler material is rapidly removed, the low spots still being protected by the cap material. Cronin et al. (U.S. Pat. No. 5,292,689 March 1994) take an entirely different approach. Silicon pillars are formed inside the trenches that are to be filled and planarized. In effect, a series of sub trenches are formed within each primary trench, these sub trenches being much less susceptible to dishing problems.